1. Field of the Invention
This invention pertains generally to EMI/RFI penetration resistant structural materials, and more particularly to a process for creating a durable EMI/RFI shield between two or more adjacent metal surfaces.
2. Description of the Background Art
Many household, industrial, commercial and military electronic devices are susceptible to interference by electromagnetic and radio frequency radiation sources in their surrounding environment. A radio frequency circuit, for example, often requires shielding to prevent electromagnetic energy generated by the circuit from interfering with the circuit or with the operation of adjacent electronic devices. Likewise, in military applications, sophisticated communications, command and logistical equipment are vulnerable to electromagnetic interference in battlefield settings. Both enemy and friendly sources of electromagnetic interference may disrupt sensitive equipment rendering it temporarily or permanently inoperative. Natural or man-made electromagnetic pulses may also significantly disrupt equipment.
Additionally, electromagnetic sources created by certain equipment may also be intercepted and used by an enemy to locate the radiation source. Similarly, transmitted communications and unencrypted data that is radiating within an enclosure or room may be intercepted by an enemy or a corporate spy allowing a loss of private or essential information. Therefore, it is essential that any enclosure or room that houses sensitive equipment or systems provide a shield to protect the equipment from damage or thwart the compromise and loss of vital information.
Electromagnetic interference (EMI) can occur at frequencies anywhere along the electromagnetic spectrum. The radio frequency part of the electromagnetic spectrum is normally considered to include the range of between 10 kilohertz (KHz) and 10 gigahertz (GHz) and is included in the term electromagnetic radiation. Radio frequency interference (RFI) and electromagnetic interference (EFI) are use interchangeably herein.
A wire or a trace on a printed circuit board, for example, can act as an emitter of electromagnetic interference by the action of moving a current through the wire that creates an electromagnetic field. A wire or trace may also act as a receptor of electromagnetic interference by exposure to an electromagnetic field. Consequently, the enclosures of sensitive electronic devices preferably shield the circuitry from outside electromagnetic interference (EMI) by either reflecting or absorbing the electromagnetic energy so that the energy in the environment surrounding the electronic devices remains at acceptable levels.
One generally known EMI/RFI shield is a housing that is electrically conductive and is electrically grounded. Current is caused to flow in a conductive barrier when exposed to an electromagnetic force field. When the electromagnetic field penetrates the conductive barrier, the current is attenuated e.g. reduced in amplitude in what is known as the skin effect. The current flow in the barrier is approximately equal to twice the magnetic field strength incident to the barrier when the field is perpendicular to the barrier and is called the surface current density. Surface current density is typically measured in amps/meter. It has also been shown that the power of the electromagnetic field as it leaves the conductive barrier is approximately equal to the impedance of the barrier times the square of the current. The power is usually determined in watts per meter squared. Accordingly, electromagnetic radiation emanating from the inside or the outside of the housing is absorbed by the conductive material and dissipated through the ground and away from the sensitive electronic components thereby permitting the proper operation of the electronic equipment.
However one deficiency with such shields is that the electrical currents created in the conductive material of the housing can be disrupted by gaps between panels or around access doors and the like and can hinder the conduction of the EMI energy to the ground. Such gaps may lead to leakage EMI energy through the shield. In some circumstances, the gaps may act like slot antennas resulting in the shield becoming a secondary EMI source. This is due to the fact that a voltage is created across the seam that is approximately equal to the current times the impedance of the seam. Reducing the impedance of the seam through the use of gaskets or the like may reduce the power radiating from the seam.
Shelters and other enclosures of electronic equipment have also been developed to provide EMI shielding for military and civilian electronic equipment. For example, an EMI shielded shelter is described in U.S. Pat. No. 6,111,192 that is collapsible for easy transport. Such shelters are typically designed to fit on the back of a truck for mobility. However, such shelters may also experience electromagnetic leakage through the joints between panels, doors, hatches and other access ways. The overall capacity of the shelter to shield electromagnetic interference can be reduced by leakage through seams between panels.
Another deficiency in the EMI shields known in the art is that the overall shielding capacity of the shelter degrades over time. This degradation is particularly apparent with shelters that are regularly transported or exposed to severe weather conditions and temperature fluctuations.
Therefore, there is a need for an EMI/RFI shield or shelter that efficiently protects sensitive electronics from interference and effectively eliminates leakage between the panels or sections of the shield. The present invention satisfies that need, as well as others, and overcomes deficiencies found in prior methods and structures.